The invention relates generally to a system and method for protecting a pressure sensing element from an ambient atmosphere whose pressure is being sensed. More particularly, the invention relates to a sensor package employing a pressure sensing element and method for manufacturing the sensor package.
Pressure sensors made of semiconductor materials are employed in a variety of applications because of their small size and compatibility with other electronic systems. Semiconductor pressure sensors or dies are generally used as pressure sensing elements in applications, such as combustion engines or in marine applications. For example, when employed in combustion engines, semiconductor pressure sensors are used to measure pressure variations in combustion fuel. However, semiconductor materials are sensitive to contamination caused by the harsh environment in such applications. Accordingly, if the surface of a semiconductor pressure sensor is exposed directly to an ambient environment whose pressure is being measured the pressure sensor may be adversely affected.
Therefore, typically pressure sensors are sealed in a metal container having a metal diaphragm which receives the pressure variations in the ambient environment and transfers it to the oils or fluids employed as a pressure transmitting medium, which in turn transfers the pressure to the pressure sensing element. This flow of pressure from the ambient environment to the diaphragm, and pressure transmitting medium and subsequently to the pressure sensor leads to measurement errors due to material mismatch. Also, due to wide temperature ranges involved in these applications, it is desirable to have a close match between the values of coefficient of thermal expansion (CTE) for the various materials involved, to prevent pressure fluctuations caused by thermal mismatch. For example, in case of an oil with high CTE, expansion of oil with temperature causes increased and undesirable stress on the diaphragm. Additionally, low CTE of the oil is desirable to reduce the errors from other components of the pressure sensor package, such as the diaphragm, from propagating to the sensing element. Hence, it is desirable to employ an oil which has a close CTE match with the other components of the assembly, such as diaphragm.
There exists a need for a suitable pressure transmitting medium which protects the pressure sensing element from the ambient environment without substantial loss in accuracy by having a close match of the CTE with other materials employed in the package.